Method for preparing side attach pad traces through buried conductive material

ABSTRACT

A circuit substrate utilizes buried edge connectors. The buried edge connectors are mechanically disposed within the edge of the substrate and have substantial thickness. The configuration and method for making the same provides relatively large edge connectors mechanically constrained in the edge of a circuit substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to circuit substrates, andparticularly to circuit substrates that utilize conductive contactsalong the sides or edges of the substrate.

2. Background of the Related Art

Circuit substrates, such as circuit boards, are electronically connectedto other components through conductive contacts disposed in or on thesubstrate. Conventionally, components were attached to contacts locatedon the top or bottom surface of the circuit substrate. The componentswere attached, for instance, by soldering conductive component leads tothe appropriate contacts.

As devices utilizing circuit substrates have decreased in size, therehas been great demand for miniaturization of the circuit substrate. Thisdemand has prompted attempts to utilize the sides or edges of thesubstrate for attachment of some components. Accordingly, the conductivecontacts to which the components are connected also have been moved tothe edges of the circuit substrate.

One approach has been to use screen printed metallization for placingconductive contacts along the edge of a given circuit substrate. Thecomponents are then attached to these conductive contacts by knownmethods.

The screen printing technique can be problematic, because themetallization at each contact is relatively thin, which can lead toexcess material leaching. The end result is loss of adhesion between theconductive contact and the attached component. Additionally, therelatively thin metallization allows for only limited reworkapplications before undue degradation of the conductive contact.

It would be advantageous to have thicker, more durable conductivecontacts along the edges of a circuit substrate to facilitate attachmentof various components thereto.

SUMMARY OF THE INVENTION

The present invention includes a method for providing metallizedconductors along the edge of a circuit board. The method includesstacking a plurality of uncured layers of a circuit substrate, andforming at least one via through the plurality of layers. The methodalso includes filling the at least one via with a metallized conductivematerial; covering the at least one via with a cover layer; andproviding a vent to the at least one via to exhaust any gases createdduring processing.

According to another aspect of the invention, a method is provided forcreating a circuit board substrate with edge connectors. The methodincludes providing a bottom layer of a green laminate. The methodfurther includes stacking a plurality of layers of the green laminate onthe bottom layer, and aligning apertures in each layer of the pluralityof layers to create a via. The method also includes filling the via witha conductive material, and connecting a vent to the via for exhaustionof byproduct gases during firing of the green laminate.

According to yet another aspect of the invention, a method is providedfor creating a circuit board with a buried edge connector. The methodincludes creating a via within a plurality of layers of a laminate. Themethod also includes filling the via with a conductive compound, ventingthe via, and curing the laminate. Additionally, the method includesdicing the laminate through the via to expose the conductive compoundalong an edge of the laminate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will hereafter be described with reference to theaccompanying drawings, wherein like reference numerals denote likeelements, and:

FIG. 1 is a top view of a circuit substrate, according to a preferredembodiment of the present invention, prior to being diced or cut fromits surrounding laminate;

FIG. 2 is a cross-sectional view taken generally along the line 2—2 ofFIG. 1;

FIG. 3 is a cross-sectional view taken generally along line 3—3 of FIG.1 after removal of the circuit substrate;

FIG. 4 is cross-sectional view, similar to that of FIG. 3, but showingan unfilled via within a circuit substrate, according to a preferredembodiment of the present invention;

FIG. 5 is a cross-sectional view similar to that of FIG. 4 but showingthe via filled with a conductive material;

FIG. 6 is a cross-sectional view similar to that of FIG. 5 withadditional layers of the circuit substrate; and

FIG. 7 is a cross-sectional view of the circuit substrate of FIG. 6following firing and dicing of the circuit substrate.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Preferred embodiments of the present invention include both a device andmethod by which buried edge connectors may be placed in circuitsubstrates, such as circuit boards. One exemplary type of circuit boardfor which this invention is particularly beneficial is an integratedseal ring (ISR) of the type used in manufacturing compact devices, suchas implantable medical devices.

As circuit substrates have undergone continued miniaturization, it hasbecome more desirable to utilize the sides or edges of the substrate forplacement of conductive contacts to which desired components may beconnected for conduction of electrical signals. The present inventionincludes a method for providing a conductive pad, typically a metallizedpad, that is buried in the edge of a circuit substrate, such as an ISR.The inventive buried contacts have added strength and longevity, whilepermitting attachment of components by traditional solder or brazingtechniques.

According to a preferred embodiment of the present invention, vias areformed in the substrate layers, e.g. the layers of “green tape,” priorto firing or curing of the circuit substrate. The green tape typicallycomprises a ceramic material, as is commonly used in the industry.

The vias are filled with a conductive material, and additional layers ofthe substrate are built up above and below the vias. Thus, theconductive material is surrounded by the green tape material. Theassembly is then fired and diced through the vias to expose theconductive, buried edge connectors.

The device and method of the present invention may be more fullyunderstood with reference to FIGS. 1 through 7. Referring first to FIG.1, a circuit substrate assembly 10 is illustrated according to apreferred embodiment of the present invention. Circuit substrateassembly 10 includes a plurality of substrate layers 12 (see FIG. 2)that are stacked to a thickness required by a desired circuit substrate14. In this particular illustrated embodiment, circuit substrate 14comprises an integrated seal ring (ISR) having an outer ring 16 and aninner recessed area 18. The circuitry for a given circuit substratedesign is disposed on at least some of the substrate layers 12, as iswell known by those of ordinary skill in the art.

In forming circuit substrate assembly 10, the individual substratelayers 12 are selectively formed with desired apertures therethrough toachieve a desired end result. For example, a plurality of alignmentholes 20 are formed through all of the substrate layers 12 external tocircuit substrate 14. Holes 20 facilitate alignment of the individualsubstrate layers 12 during assembly and during the firing or curing ofcircuit substrate assembly 14.

Additionally, selected substrate layers 12 are formed with one or morevia openings 22. Openings 22 are located such that when substrate layers12 are stacked above one another, via openings 22 form a desired via 24.For example, a plurality of vias may be formed along an edge or edges 26of circuit substrate 14, as best illustrated in FIGS. 1-3. Otheropenings also may be formed in selected substrate layers 12 to createvarious features, such as inner recessed area 18, of circuit substrate14.

Following curing, i.e., firing, of the circuit substrate assembly 10,circuit substrate 14 is removed. This may be accomplished by cuttingcircuit substrate 14 from the remainder of circuit substrate assembly10. The cutting, commonly known as dicing, is performed along theperimeter of circuit substrate 14 as illustrated by lines 28A through28D in FIG. 1. The dicing along line 28A passes directly through vias 24to expose each buried edge connector 30. As will be explained more fullybelow, each buried edge connector 30 is formed from a conductivematerial inserted into one of the vias 24 prior to firing of circuitsubstrate assembly 10.

It should be noted that via openings 22 and vias 24 may be formed in avariety of shapes and configurations. In the embodiment illustrated inFIGS. 1-3, via openings 22, and therefore the cross-section of each via24, is square or rectangular. It has been determined that thissquare/rectangular shape helps to securely hold each buried edgeconnector 30 within its corresponding edge 26. The design provides amechanical mechanism 31, in the form of edges 32, that physically gripseach buried edge of the connector 30 to prevent its detachment. Themechanical gripping provided by edges 32 increases the durability ofeach connector 30 because its attachment does not simply depend onadhesion of materials. There also may be other cross-sectional shapesthat provide mechanical assistance in securely holding each buried edgeconnector 30 along its corresponding edge 26.

The present invention allows vias 24 to have a greater than expecteddiameter, e.g. thickness, (see diameter 33 illustrated in FIG. 1). Formany fill materials used in filling other types of vias, it isrecommended that the diameter 33 of the vias not exceed two times thethickness of a substrate layer 12. The present invention permits thefilling of buried edge connector vias having a diameter greater thantwice the individual layer thickness. Typically, the diameter of the viacan be substantially greater than twice the substrate layer thicknesswithout creating detrimental effects with respect to the edge connector.This permits the burying of larger edge connectors within edge 26 ofsubstrate layers 12 to provide a stronger, more durable circuitsubstrate 14.

The formation of circuit substrate 14, according a preferred method ofthe present invention, can be described with reference to FIGS. 4-7.Referring first to FIG. 4, a plurality of substrate layers 12 arestacked on top of one another. At this stage, substrate layers 12 aregreen (unfired) and form a green laminate 34. (The portion illustratedin FIG. 4 shows a cross-sectional view of only a portion of the overallgreen laminate 34 that ultimately forms circuit substrate assembly 10.)

The portion illustrated includes an exemplary via 24 formed of multiplevia openings 22 disposed through individual substrate layers 12.However, the description herein applies equally to the other vias 24 andcircuit substrate assembly 10 as a whole. In fact, during manufacture ofcircuit substrate 14, the vias 24 typically are filled and processedsimultaneously.

Disposed beneath via 24 is at least one substrate layer 12 that does notinclude a via opening 22. Preferably, only a single substrate layer 12is disposed beneath the via or vias 24 at this stage in the process. Forclarity of description, the bottom substrate layer 12 shall be referredto as layer 12′.

Bottom layer 12′ and the remaining green substrate layers 12 of greenlaminate 34 are placed on a base plate 36 of a via fill machine (notshown). Via fill machines are known to those of ordinary skill in theart and may be purchased, for example, from Pacific TrineticsCorporation of Carlsbad, Calif. 92008. Via fill machines typicallyutilize a vacuum applied to the vias to move desired fill material intothe via.

Preferably, a sheet of relatively porous paper 38 is placed between baseplate 36 and bottom layer 12′. When green laminate 34 is positioned onpaper 38 and base plate 36, a vacuum is applied at base plate 36 tofacilitate filling of via 24 with a conductive material 40. Paper sheet38 and bottom layer 12′ cooperate to ensure application of an evenvacuum to via 24 so that conductive material 40 is forced throughout via24.

The above-described method of filling vias 24 often is referred to as anextrusion process. When a sufficient amount of conductive material 40 isdisposed above a desired via 24 and a vacuum is created in the via 24,the external pressure is utilized to effectively extrude the conductivematerials throughout the via 24. An air impermeable sheet may be placedover the conductive material 40 prior to extruding it to further takeadvantage of the pressure differential created by the vacuum applied atvia 24. Regardless of the specific method used to fill via 24, it isdesirable to have conductive material 40 fully dispersed throughout thevia.

Furthermore, the extrusion process potentially can be carried out withor without paper sheet 38 and with additional bottom substrate layers12′. However, the inventors have determined that the natural porosity ofa single green layer of substrate combined with a paper sheet providesadequate vacuum with sufficient disbursement of the vacuum to facilitatefilling of via 24.

Conductive material 40 usually is a metallized conductive material. Thematerial chosen should have appropriate solder acceptance qualities, andits percent shrinkage during firing should be compatible with theshrinkage of the green substrate layers 12 during firing. A preferredconductive material is Platinum-Gold Via Fill Material for the GreenTape System, Product Number 5739Y, available from DuPontMicroelectronics.

After via or vias 24 are filled, additional green substrate layers 12(labeled 12″ for clarity) are disposed beneath bottom substrate layer12′. Similarly, additional green substrate layers 12 (labeled 12′″ forclarity) are disposed above the filled via 24 to substantially enclosevia 24 and complete green laminate 34. A vent 42 is formed through greenlaminate 34 to permit escape of gases from conductive material 40 andvia 24. Without vent 42, the substrate layers 12 as well as the finishedburied edge connectors 30 may incur damage, such as blistering, due totrapped gases during the firing process.

In the preferred embodiment, vent 42 is formed through the greensubstrate layers 12′″ disposed above via 24. Additionally, vent 42preferably is located external to line 28A along which the substrate isultimately cut, so that the vent is completely removed followingcompletion of circuit substrate 14. Vent 42 may comprise one or morevents to each via 24 depending on the size of the subject via 24, thesize of each vent or vents 42 and the type of conductive material usedto fill via 24.

After completion of the green laminate 34 as illustrated in FIG. 6, itis fired and diced, as illustrated in FIG. 7. Following the dicingoperation, the circuit substrate 14 is separated for use in a desireddevice.

As will be apparent to those of ordinary skill in the art, the presentinvention is directed to buried edge connectors and the method ofsuccessfully forming durable buried edge connectors. The actual layoutof the circuitry on the circuit substrate 14 and the connection of thatcircuitry to the buried edge connectors 30 varies depending on theultimate application of circuit substrate 14. The circuit layouts andconnections to buried edge connectors 30 can be accomplished astraditionally done and understood by those of ordinary skill in the art.

It will be understood that the foregoing description is of a preferredexemplary embodiment of this invention and that the invention is notlimited to the specific form shown. For example, the present inventivemethod lends itself to a variety of circuit substrates other than ISRs;various materials, used now or in the future, can be implemented to formthe layers of the substrate; various metallized and non-metallizedconductive materials may be used to form the buried edge connectors; thesize and shape of the vias can be modified according to the desiredapplication; and the buried edge connectors can be disposed along one ormore edges of the substrate. These and other modifications may be madein the design and arrangement of the elements without departing from thescope of the invention as expressed in the appended claims.

What is claimed is:
 1. A method for providing metallized conductorsalong an edge of a circuit board, comprising: stacking a plurality oflayers of a substrate used in formation of the circuit board; forming atleast one via through the plurality of layers; filling the at least onevia with a metallized conductive material; covering the at least one viawith a cover layer; and providing a vent to the at least one via toexhaust any gases created during processing; dicing the plurality oflayers through the at least one via at a position offset from the ventmetallized conductive material is exposed which is substantiallyperpendicular relative to a major surface of the plurality of layers. 2.The method as recited in claim 1, further comprising firing theplurality of layers and the metallized conductive material.
 3. Themethod as recited in claim 1, further comprising placing a bottom layerto the substrate beneath the at least one via.
 4. The method as recitedin claim 3, further comprising applying a vacuum to the bottom layerduring filling of the at least one via.
 5. The method as recited inclaim 1, wherein providing a vent includes forming the vent through thecover layer.
 6. The method as recited in claim 1, wherein forming atleast one via includes forming a plurality of vias in general linearalignment.
 7. The method of claim 1, wherein forming at least one viacomprises forming at least one aperture through each of the plurality oflayers, the at least one aperture having a diameter that is greater thantwice a thickness of one of the plurality of layers.
 8. The method ofclaim 1, wherein stacking a plurality of layers of a substrate comprisesstacking a plurality of layers of a green laminate.
 9. The method ofclaim 1, wherein providing a vent comprises providing a separate vent toeach of the at least one via.
 10. The method of claim 1, whereinproviding a vent comprises providing an aperture through the cover layerwhich is offset from a centerline of the at least one via.
 11. A methodfor providing metallized conductors along an edge of a circuit board,comprising: stacking a plurality of layers of a substrate used information of the circuit board; forming at least one via through theplurality of layers; filling the at least one via with a metallizedconductive material; covering the at least one via with a cover layer;providing a vent which extends through the cover layer and which is opento a top surface of the metallized conductive material within the atleast one via which does not extend through the at least one via; andcuring the substrate, wherein any gases created in the at least one viaduring curing are exhausted through the vent.
 12. The method of claim11, further comprising dicing the plurality of layers through themetallized conductive material to expose a surface of the metallizedconductive material which is substantially perpendicular to a majorsurface of the layers.
 13. The method of claim 11, wherein stacking aplurality of layers of a substrate comprises stacking a plurality ofuncured layers of a green laminate substrate.
 14. The method of claim 11further comprising placing a single layer of the substrate beneath theat least one via and applying a vacuum to the single layer to fill theat least one via with the metallized conductive material.
 15. The methodof claim 11 wherein providing a vent comprises providing a separate ventto each of the at least one vias.